1. Field of the Invention
This disclosure sets out a system to be used with a drilling rig, fluid system, and more particularly a system for removing undesirable cut from the drilling fluid.
2. Background of the Invention
When an oil well is drilled, it is necessary to drill the well with drilling fluid. The drilling fluid is provided to lubricate the drill bit, and carries away cuttings by flowing upwardly in the annular flow space around the drill string. The drilling fluid is pumped down the drill string to pick up the cuttings and other debris. Commonly, the drilling fluid is water but it is sometimes made with an oil or oil based carrier. Generally, various heavy metal or other minerals are added to give it a weight and a selected viscosity. The viscosity is obtained from clay or clay products. The drilling mud becomes slick to the touch so that it provides a lubricating benefit.
When drilling into a high pressure formation, safety is enhanced by incorporating a weight component to the drilling mud. Common weight additives are barium sulfate. Water has a weight of about 8.4 pounds per gallon. The weight of the drilling mud can be increased to as much as 17 or 18 pounds per gallon by adding the weight materials. Occasionally, higher weights are achieved also by addition of these or other added weight materials. The weight materials have a relative density of around 4.0 compared to water (a density of 1.0).
After circulation through the well, the drilling mud will pick up particles of the earth formations that are penetrated by the drill bit. It is a relatively easy thing to clean the drilling mud provided the cuttings are primarily heavy rock. Also, the cuttings are easily removed where they form large particles. They can be readily removed when the drilling mud is recycled by passing the drilling mud through a set of screens. The mud is recycled at the surface on return from the well borehole by flowing up through the annular flow space around the drill string. It flows out of the annular flow space into a mud pit. Ultimately it is recycled back through the mud pump. While flowing from the well to the mud pit and then back to the pump, the mud typically is treated by a number of devices to restore the mud to the original condition. The mud is transferred through any of several devices including degassers, shale shakers, desanders and other cleaning devices. At times, the mud will simply be permitted to sit in an open pit. This enables the heavy particles in it to settle to the bottom. The mud is cleaned by settled heavy particles. Gas bubbles also are removed so that entrained gas bubbles do not create a risk of explosion by accumulating odorless natural gas around the mud pits. Gas cut mud is too light; the mud weight may be deceptively light.
In many ways, separation techniques applied to the drilling mud run into problems because of the difficulty of separating the desirable added components along with those retrieved from the well. The present disclosure is a method and apparatus for selective removal of trash or debris in the mud stream. The mud flow is made of several components including water which has a specific gravity of about 1.00 (assumed to be fresh water), and weight materials added to it which have a specific gravity of about 4.0. Sometimes, and dependent on the nature of the formation penetrated, the mud will be commingled with cuttings from sand and shale formations (a specific gravity of about 2.6). Sand cuttings are more easily removed. Shale cuttings however are difficult in that the shale is derived from formations incorporating a low grade clay. The specific gravity 2.6 of the shale formations is such that it is difficult to sort or separate. It cannot be wholly removed by sieves or screens. It is not in the form of particles sufficiently large that such screens will catch the particles. Moreover, it is dissolved and enters the solution so that no amount of mechanical screening or filtration can remove it. That creates a problem in recycling the mud. The drilling fluid therefore has to be replaced. Replacement of the drilling fluid is quite expensive.
In drilling a well, and especially a deep well, the problems are minor at shallow depth and become notable with depth. Typically, the first several hundred feet of drilling will be accomplished in just a day or so and is drilled rather rapidly. The problem arises at greater depths where the drill bit penetrates several formations of shale. The clay that is in the shale will dissolve, thereby changing the physical characteristics and performance of the drilling mud. Mud will no longer exhibit the integrity necessary for continued reuse. As the drilling mud is cut with added well bore materials, it ultimately is necessary to dispose of the entire batch. At that point, the well is quite deep, and the amount of mud required for replenishment can be several hundred barrels, indeed as much as 2000 barrels of fluid. This is expensive with a water based mud. It is even much more expensive with an oil based mud. It is not uncommon to have as much as $1,000,000 worth of drilling fluid solvents mixed into the drilling fluid and in circulation in a well at that moment. Some drilling fluids cost as much as $300 per barrel in 1998 prices. Easily, a single well can tie up $1,000,000 worth of drilling fluid at a given moment. The life of the drilling fluid is protected and extended by the present apparatus. Moreover, the method of sorting out the components that are desirable from those that are undesirable is set forth.